A defibrillator is a device that stores energy, typically in one or more high-voltage capacitors, and delivers the stored energy to a patient. In particular, a defibrillator delivers energy to a heart that is undergoing fibrillation and has lost its ability to contract. Ventricular fibrillation is particularly life threatening because activity within the ventricles of the heart is so uncoordinated that virtually no pumping of blood takes place. An electrical pulse delivered to a fibrillating heart may depolarize the heart and cause it to reestablish a normal sinus rhythm. For some patients, more than one defibrillation pulse is required.
An external defibrillator applies a defibrillation pulse via electrodes placed upon the patient's chest. When a switch is closed, current flows between the electrodes and the defibrillator delivers at least some of the stored energy to the patient's chest. The dosage of energy delivered may be on the order of two hundred joules or more, but the dosage depends upon the circumstances. The quantity of energy delivered when the patient is a child, for example, is generally less than when the patient is an adult. In some cases, a patient may need multiple shocks, and different dosages may be delivered with each shock.
The energy delivered to the patient, and the current that flows between the electrodes, are a function of the voltage between the electrodes, the impedance of the body of the patient and the pulse width of the shock. For a given voltage and pulse width, a patient having a body with a lower impedance will experience a different current flow, and will thus receive a different quantity of energy, than a patient having a body with a higher impedance. A defibrillator that measures the patient's impedance accurately can therefore more effectively develop a voltage across the electrodes that accurately delivers a desired dosage of energy or current to the patient. A defibrillator that measures the patient's impedance accurately can also adjust the shape of the pulse for enhanced effect.
A defibrillator may measure the impedance of the patient's body by applying an excitation current, or “carrier,” to the patient via the electrodes placed upon the patient's chest, and measuring the response to the application of the excitation current via the electrodes. The response is typically measured as a function of the voltage difference between the electrodes.
The excitation current is an alternating current signal, having a small current magnitude, such as 100 microamperes, and a known frequency. By measuring the magnitude and phase of the response, the patient's impedance can be determined. Because the patient's body is not purely resistive but includes a reactive component, the measured impedance varies depending upon the frequency of the excitation current. Defibrillators that measure impedance with an excitation current typically employ one or two particular fixed frequencies, such as 20 kHz, 30 kHz or 62 kHz.
The excitation current does not deliver the defibrillation shock. A typical defibrillation shock is of a substantially higher amperage than the excitation current. During delivery of defibrillation shocks, the impedance exhibited by the patient and “seen” by the defibrillator varies. Impedance varies from patient to patient, and may also vary within a single patient as a function of factors such as the magnitude of the defibrillation shock. The impedance exhibited by the patient during delivery of a defibrillation shock may not be the same as the impedance exhibited during measurement of the response to the excitation current.